Reduced spectral density mapping for proteins: Validity for studies of C-13 relaxation

Spectral density mapping provides direct access to protein dynamics with no assumptions as to the nature of the molecule or its dynamic behaviour. Red uced spectral density mapping characterises a protein's motions at a lower experimental burden, assuming that the spectral density function J(omega) i s flat around omega(H). This introduces little error for N-15 relaxation da ta but is less valid for C-13 studies, perturbing J(omega(C)) considerably to an extent that depends on the nature of the molecule's motions. We propo se the fitting of spectral density at high frequencies to a single Lorentzi an and show that the true values of the spectral density lie between those determined by the two approximations.